The demands placed upon the saccadic system change throughout life as a result of maturation (development/aging), injury, or disease. It appears that the saccadic system has evolved a mechanism that can adjust its performance in response to these demands. I believe such improvements in performance reflect a neurobiological process called adaptive motor learning. To study adaptive motor learning in the saccadic system I have developed a psychophysical procedure that introduces a "visuomotor mismatch" between the retinal error signal (retinal distance between fovea and target image) and the motor error signal (movement required to accurately foveate the target). The saccadic system responds to this viruomotor mismatch by rapidly modifying the amplitude of the saccade. I will refer to this procedure as Experimentally Induced Saccadic Dysmetria because it accurately mimics the errors that the visual and oculomotor system would incur during ordinary dysmetria, e.g., when the saccadic system has been weakened. In this paradigm, the dysmetria is produced by electronically adding, or subtracting, a portion of the eye movement signal to adjust the target position. Thus the original visual error signal that initiates the saccade no longer elicits an appropriate sacade; there is now a mismatch between the target step and the saccade amplitude. The advantages of this particular paradigm over others that have been developed is that 1) it is rapid and reversible, 2) it produces a highly-controlled visuomotor error, 3) it can be used in intact monkeys and humans, and 4) it used normal visual and oculomotor pathways. Experimentally Induced Saccadic Dysmetria provides us with a powerful tool for the study of adaptive motor learning in the oculomotor system. As a first step the proposed behavioral experiments will study 1) the circumstances during which saccadic adaptation can occur and 2) the characteristics of the adapted saccades. The results of these experiments will provide information concerning (1) factors controlling the magnitude and time course of the adaptive effects and (2) the neural structures that participate in the adaptive process.